Hi Manfred,
I don't have much to offer because you don't have one of the ancient Q meters but with these old instruments you could determine the Q of a cap.
I also have a GR-821 A Twin T Impedance Measuring Circuit that can determine the Q from 0.5 to 30 MHz. It measures the capacitance and the conductance.
Also, I have the old GR1608A that can directly measure the D factor at 1 kHz and with a simple correction factor it can measure the D factor at other frequencies from 20 Hz to 20 kHz.
73
